Collaborative Research: Geochemistry of the Source Regions of Southwest Pacific Volcanic Islands: An Approach Using Ultramafic Xenoliths & Host Basalts

这是斯克里普斯研究所的James Natland和麻省理工学院的Fred Frey的合作项目。它是对来自西南太平洋萨摩亚群岛、社会群岛和马克萨斯群岛的超镁质捕虏体及其宿主熔岩的综合结构、矿物学和地球化学研究。这些捕虏体存在于现有的集合中，初步的矿物学和同位素研究表明，它们与火山下的部分熔融和岩浆分异过程有关。岛链包含一些海洋盆地中最丰富的玄武岩熔岩，这被假设与地幔中区域广泛的，也许是半球的地球化学异常有关。捕虏体应提供关于地幔源区组成、交代过程在影响或改变源区组成方面的作用、洋岛玄武岩源区及其附近的熔体提取和岩浆分异过程的补充资料。迄今为止的研究表明，在所有三个岛链中都存在残余（1型）和“累积”（2型）捕虏体，并且相当程度的部分熔融产生了可能的拉斑岩岩浆，这些岩浆在从地幔出发的过程中结晶了橄榄石-尖晶石-辉石组合。这些熔岩可能包含了这些火山中大量的、被淹没的、但仍未取样的部分。应用于捕虏体的主要技术将是：1)岩石学；2)矿相电子探针显微分析；3)矿物中痕量元素丰度的离子探针分析，4)质谱分析（传统固体源和ICP-MS）， XRF和仪器中子活化分析的组合，用于整个岩石和矿物分离。步骤1和步骤2将用于选择适当范围的样品进行详细的离子探针、同位素和微量元素研究。将对宿主熔岩进行大量化学、微量元素和同位素测量，这些测量与岛上的时间演替有关，这是已知的。还将对大陆和夏威夷捕虏体套件的出版资料进行比较。

This is a collaborative project between James Natland of Scripps Institution, and Fred Frey of MIT. It is an integrated textural, mineralogical, and geochemical study of ultramafic xenoliths and their host lavas from the Samoan, Society, and Marquesas Islands in the southwest Pacific. The xenoliths are in existing collections, and preliminary mineralogical and isotopic work demonstrates that they are related to processed of partial melting and magmatic differentiation beneath these volcanoes. The island chains contain some of the most enriched basaltic lavas in the ocean basins, which are hypothesized to be related to a regionally widespread, perhaps hemispheric, geochemical anomaly in the mantle. Xenoliths should provide complementary information on the compositions of mantle sources, the action of metasomatic processes in influencing or modifying source compositions, and the processes of melt extraction and magmatic differentiation in and near the source regions of ocean-island basalts. Studies so far indicate that both residual (Type 1) and "cumulate" (Type 2) xenoliths are present in all three island chains, and that substantial degrees of partial melting produced possible tholeiitic magmas which crystallized olivine-spinel- pyroxene assemblages en route from the mantle. Such lavas may comprise much of the voluminous, submerged, but still unsampled portions of these volcanoes. Principal techniques to be applied to the xenoliths will be 1) petrography; 2) electron-probe microanalysis of mineral phases; 3) ion-probe analysis of trace-element abundances in minerals, and 4) a combination of mass spectrometry (conventional solid- source and ICP-MS), XRF, and instrumental neutron-activation analysis for whole-rocks and mineral separates. Steps 1 and 2 will be used to select the appropriate range of samples for detailed ion-probe, isotope, and trace-element studies. Bulk chemical, trace-element, and isotopic measurements will be made on host lavas and these related to temporal successions on the islands, which are already known. Comparisons will also be made to published information on continental and Hawaiian xenolith suites.